Nobel metal chalcogenides are widely known in the field of electrocatalysis; in particular, electrocatalysts based on rhodium and ruthenium sulfide are currently incorporated in gas-diffusion electrode structures for use as oxygen-reducing cathodes in highly aggressive environments, such as in the depolarized electrolysis of hydrochloric acid.
Nobel metal sulfides for use in electrocatalysis are prepared by sparging hydrogen sulfide in an aqueous solution of a corresponding noble metal precursor, usually a chloride, for instance as disclosed in U.S. Pat. No. 6,149,782 which is relative to a rhodium sulfide catalyst. The synthesis of noble metal sulfide catalysts with hydrogen sulfide in aqueous solutions is conveniently carried out in the presence of a conductie carrier, in most of the cases consisting of carbon particles. In this way, the noble metal sulfide is selectively precipitated on the carbon particle surface, and the resulting product is a carbon-supported catalyst, which is particularly suitable for being incorporated in gas-diffusion electrode structures characterized by high efficiency at reduced noble metal loadings. High surface carbon blacks, such as Vulcan XC-72 from Cabot Corp./USA are particularly fit to the scope.
A different procedure for the preparation of carbon-supported noble metal sulfide catalysts consists of an incipient wetness impregnation of the carbon carrier with a noble metal precursor salt, for instance, a noble metal chloride, followed by solvent evaporation and gas-phase reaction under diluted hydrogen sulfide at ambient or elevated temperature, whereby the sulfide is formed in a stable phase. This is, for instance, disclosed in the co-pending provisional application Ser. No. 60/473,543, which is relative to a ruthenium sulfide catalyst.
In the case of rhodium, prior to its use, the noble metal sulfide catalysts so obtained are subjected to an adequate stabilizing thermal treatment, at a temperature usually between 300° and 700° C. In other cases, a temperature as low as 150° C. may be sufficient for an adequate thermal treatment.
Although these catalysts show good performances in terms of oxygen reduction activity and of stability in highly aggressive environments, that makes them virtually the only viable materials for oxygen reduction catalysts in hydrochloric acid electrolysis, their production via hydrogen sulfide route is affected by some inconveniences.
Firstly, the use of a highly hazardous species such as hydrogen sulfide, which is a flammable and noxious gas, in their synthesis poses serious environmental and human health concerns. The handling of hydrogen sulfide is a very delicate matter which can only be dealt by resorting to expensive safety measures.
Secondly, the precipitation in an environment where free sulfide ions are present can lead to the formation of compounds with variable stoichiometry, and this can hamper the reproducibility of the required catalyst, especially with certain noble metals; sulfide ions are furthermore a toxic and environmentally unfriendly species.
Other common reagents for the precipitation of sulfides, such as polysulfides, thioacetic acid or thioacetamide, are less hazardous than hydrogen sulfide, but the reaction pathway in an aqueous environment still follows a pre-ionization or hydrolization of these compounds to provide undesired free sulfide ions.
An alternative synthetic route for the production of noble metal sulfides to be used in oxygen reduction catalysts, in the absence of free sulfide ions and especially of the highly flammable and highly toxic hydrogen sulfide species is therefore a stringent requirement for a successful scale-up of noble metal sulfide catalyst production, and eventually for the commercialization of potentially large electrochemical processes such as the depolarized electrolysis of hydrochloric acid.
These and other objects and advantages of the invention will become obvious from the following detailed description.